The invention is in the area of tertiary oil recovery techniques, in particular, an improved apparatus for downhole injection of steam into boreholes.
In the art of recovering oil from earth formations, tertiary methods are increasing in their importance. Initially, oil flow from many wells is driven by the pressure due to natural gases trapped along with the liquid oil in the formation. With the passage of time, natural gas pressures decrease. When gas pressure is insufficient to drive oil to the surface, pumping methods are then employed. As time passes, pumping methods may be ineffective because the flow of oil underground out of porous formations into a well may be very slow. It is at this point that tertiary methods are sought to accelerate the flow of oil from the formation into the well. A particularly useful tertiary method employs the injection of steam. Steam serves to heat the oil in the formation, thereby reducing its viscosity and increasing its flow rate into the well for recovery.
Methods employing downhole generation of steam within a well have proved to be particularly advantageous. The prior art discloses several representative methods and apparatus.
In U.S. Pat. No. 3,456,721, Smith discloses a downhole burner for generating steam. Gaseous or liquid fuels are mixed with air and combusted in a burner with simultaneous spraying of water toward the flame. The water is sprayed from a cylindrical water jacket through a plurality of orifices. Steam is formed by the vaporization of the water as the water bombards the flame.
In U.S. Pat. No. 3,980,137, Gray discloses a downhole steam injector employing the combustion of hydrogen with oxygen to generate heat to vaporize injected water to form steam. The water moves through an annular jacket surrounding the combustion chamber and, after being preheated, enters the combustion chamber through plurality of grooves or passages at the top of the combustion chamber near the ignitor and the hydrogen/oxygen flame.
Hamrick and Rose in their related U.S. Pat. Nos. 3,982,591 and 4,078,613 disclose downhole steam generators. In the first patent, in FIG. 17, water is injected through a plurality of apertures directly into the flame in a hydrogen/oxygen combustion zone. In the second patent, in FIG. 2B, water moves into a cooling annulus before it is injected into a mixing zone spaced below the combustion zone. The mixing zone is defined by a cylindrical wall which has a plurality of apertures through which water from the cooling annulus passes laterally into the mixing zone. A heat-resistant liner is placed along the interior of the combustion zone.
In the latter patent, methane is burned in oxygen in two stages to reduce the formation of soot. Firstly, gaseous methane is compressed, carried down into the borehole and reacted with oxygen in a controlled manner to form carbon monoxide and hydrogen. Subsequently, carbon monoxide and hydrogen are reacted with oxygen toform carbon dioxide and water.
Several problems have been encountered with these prior art downhole steam generators. A particularly dangerous problem relates to burnback; that is when the combustion zone extends back into the precombustion area. Burnback is caused by several contributing factors which include: overheating of the boundary layer adjacent the inner wall of the precombustion zone; a boundary layer which is thick and of low velocity; and thermal conduction from the combustion zone to precombustion areas.
Another problem with the prior art downhole steam generators is the tendency of the flame in the combustor to be quenched by water injected directly into the flame.
An additional problem related to the direct injection of water into the combustion zone is wetting the sparkplug or ignitor, thereby reducing the efficiency of combustion.
A problem related to inefficient combustion is the formation of soot. Soot has two deleterious effects. One is air pollution, and the other is the tendency to clog the pores in the earth formation, thereby impeding oil flow out of the formation into the production well.
The disclosure of suitable fuel forms in the prior art is limited to fuels which are: high pressure fluids which are gases at ambient temperature; and liquids which are atomized prior to burning. Absent from this list are liquid fuels which are in the liquid state at high pressure and ambient temperature and which are gaseous at high pressure and elevated temperature conditions. Pressurization of gases requires elaborate equipment which is not necessary when employing a liquid fuel.
Another problem prevalent with the prior art devices employing heat resistant combustion zone liners is that the liners are not cooled adequately by adjacent heat transfer jackets. As a consequence, the liners cannot withstand the prolonged high temperatures of the combustion zone and undergo severe deterioration.